This application relates to a process for removing contaminated sediments from the bottom of lakes, reservoirs, rivers, streams, and other water bodies, while at the same time preventing the release of particulate matter or soluble contaminants to the ambient water environment during the dredging excavation process.
Removal of sediments that accumulate on the bottom of natural and artificial water bodies is commonly practiced to permit navigation of ships and/or to maintain designated water depths. This type of sediment removal is commonly referred to as maintenance or navigational dredging. Sediments are sometimes removed to clean up the bottom of these water bodies when such sediments are found to be contaminated and pose a threat to public health and/or the ecosystem. This type of sediment removal is commonly referred to as environmental dredging. In some cases the objectives of the sediment removal activity are both maintenance and environmental.
Current methods of maintenance or environmental dredging can be divided into two general categories. They include mechanical dredging and hydraulic or vacuum dredging. The fundamental difference between these categories is the equipment used and ultimately the form in which the sediments are removed. Mechanical dredges typically remove the sediments directly with clamshell-type buckets at a relatively low liquid to solid ratio (i.e., relatively little water is entrained in the sediments compared to hydraulic dredging operations). Hydraulic or vacuum type dredges agitate the bottom channel to dislodge the sediment, and pump (vacuum) the sediment from the waterway. In hydraulic dredging operations the sediment is transported in a slurry with water acting as the transportation medium. This results in a water sediment mix with a high liquid to solid ratio. The sediment in the slurry must later be segregated from the water carrier. This is typically accomplished using large impoundment areas where the sediment is extracted by settling and the water (effluent) is returned to the originating waterway.
The removal of bottom sediments, whether by a mechanical or by a hydraulic dredging operation, will involve some form of raking, grabbing, penetrating, cutting, or hydraulically scouring the bottom of the waterway or channel. During such operations, sediments will readily be dispersed. In addition to sediment loss due to resuspension, in the case of mechanical dredging operations, sediment loss will occur when the bucket leaks sediments, and particularly when the bucket is raised above the water during the excavation process. This leakage occurs due to improper bucket closure resulting from debris stuck in the bucket, inadequate bucket sealing mechanisms, and the displacement of water contained within the bucket that occurs when solids enter the bucket during the excavation. While hydraulic dredging operations may have the advantage of a vacuum system that can assist in capturing some resuspended solids during bottom scouring operations, the large volumes of water that must be withdrawn and processed during such operations limit the feasibility of hydraulic dredging operations to areas where large impoundments are available. In addition, the presence of tides and currents can be expected to significantly reduce the efficiency of capture of resuspended solids by vacuum dredges when compared to operations that occur in quiescent waters.
Sediment resuspension and loss during dredging is a particular concern in environmental dredging operations where sediments are contaminated and the resuspension and dispersion of such sediments can result in ecological and human health impacts. This concern is underscored by the fact that most contaminants are generally associated with or bound to the fine particles, which are also those particles that are most easily resuspended and dispersed during the dredging operation. In addition to particulate resuspension, the potential release of soluble contaminants that may be present in the pore space of contaminated muds or may be subject to dissolution from the mud particle upon resuspension during dredging operations is also a concern.
Other problems associated with environmental dredging operations include the lack of suitable methods to monitor the actual loss of sediment that occurs during the excavation process, the lack of appropriate methods to monitor and ensure that the cleanup is being properly effected, and the absence of suitable methods to make certain that the handling of such sediments, during marine-to-land transfer and land-based transfer of such materials, do not result in liquid leakage or loss of sediments.
Current approaches for monitoring sediment loss during the excavation typically involve the use of discrete upgradient and downgradient subsurface sampling stations. Water samples collected at these stations are used to assess the increase in solids or turbidity loading to the waterway during the excavation. Given the unpredictability of subsurface currents, the discontinuous dredging operation, and discrete (spike) loadings that can be expected during dredging operations, the collection of representative samples is difficult. In addition, a determination of sediment loss can only be made after the release has already occurred.
To achieve target cleanup goals at a contaminated sediment site, due to the resuspension and the redeposition of sediment that occurs during conventional mechanical and hydraulic dredging operations, second and third passes to clean the contaminated area are routinely common. Even with multiple passes of a contaminated area, targeted specifications are still difficult to achieve. Current methods to assess the effectiveness of the cleanup of the subsurface sediments after the excavation typically involve the collection of core samples at discrete locations in the dredge area. Due to the expected variability in the spatial distribution of contamination in bottom sediments, particularly after the sediment is disturbed in a dredging operation, the collection of representative bottom sediment samples is also a difficult proposition. If the remediation effort has not met the required specifications, dredging equipment must be remobilized and returned to the location for additional cleanup at significant expense.
The removal and management of contaminated sediments during environmental dredging operations is best accomplished by collecting the sediments in a secure manner, dewatering the sediments, and transferring the sediments in such a way that there will be little risk for spills or loss of material. In mechanical dredging operations, sediments are typically placed in solids barges that require off-loading facilities that make use of additional cranes and buckets to remove the sediments from the solids barge for land-based management. Such operations are extremely messy and difficult to manage, and present a relatively high risk for further environmental contamination. Hydraulic dredging operations, as previously noted, require the construction of facilities for collecting the slurry and segregating the solids from said slurry, and thickening and/or dewatering the solids prior to transport to the disposal site. These facilities must be constructed in close proximity to the dredging operation and also increase the risk of local environmental contamination.
Most of the advances in environmental dredging technology in recent years have focused on the development of improvements in the design of buckets or vacuum dredges that tend to reduce or control the disturbance of the bottom of the waterway during the sediment excavation process (Ouwerkerk, R. and H. Greve (1994). xe2x80x9cDevelopments in Dredges During the Last Decade.xe2x80x9d Pages 690-699 in Dredging ""94, Proceedings of the Second International Conference on Dredging and Dredged Material Placement, Edited by: E. C. McNair, Jr., American Society of Civil Engineers. 1994; Zappi, P. A. and D. F. Hayes. xe2x80x9cInnovative Technologies for Dredging Contaminated Sediments.xe2x80x9d Improvement of Operations and Maintenance Techniques Research Program, U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss. Miscellaneous Paper EL-91-20. Sep. 1991; and Herbich, J. B. Handbook of Dredging Engineering, McGraw Hill, Inc., New York. 1992).
The primary objective of most mechanical bucket modifications has been to create as tight a seal as possible on the bucket through the installation of rubber backing compressible seals and sensors (proximity switches) to ensure the bucket is closed prior to lifting in the hope of minimizing spillage. A dredging apparatus referred to as a xe2x80x9ccable arm clamshell bucket,xe2x80x9d advertised by Cable Arm Incorporated of Trenton, Mich., claims to reduce turbidity levels through its use of electronic sensors to detect bucket closure and compressible seals. While these types of systems offer improved designs, they do not eliminate the impact due to raking, upswell, and water displacement that will occur on excavation. In addition, such systems are incapable of effectively addressing leakage that will occur when debris is caught in the bucket and prevents full bucket closure.
Numerous modifications and subcategories of hydraulic dredges have been developed to attempt to mitigate problems associated with hydraulic dredging particle resuspension (Cleland, J. Advances in Dredging Contaminated Sediment, Scenic Hudson, Inc., 1997). Many have specific features that are designed to reduce sediment dispersion resulting from cutterblade, jetting, or raking mechanisms. None of these methods, however, have proven to be highly effective in areas where currents are present, nor do they significantly reduce or have suitable provisions for managing large volumes of contaminated water that are generated in the process. Large impoundment areas and suitable treatment methods are still needed to contain fine contaminated particulates that are drawn up with these sediments.
In addition to the use of improved mechanical or vacuum dredge systems, physical barriers such as silt curtains (flexible, impermeable canvas or rubber-like sheets that are deployed by attaching ballast to the bottom of the fabric and floats to the top to hold the curtain in a vertical configuration) or sheet piles have been employed during environmental dredging operations in attempts to isolate the area of dredging and contain contaminated particulates that are dispersed into the water column during the excavation process. They do not prevent the migration of soluble contaminants. While these barriers provide some particulate containment, silt curtains lack secure containment due to the porous nature of joints and curtain underflow, and their use in waterways with currents or tides is, in most cases, impractical. The application of fixed sheeting along the periphery of a dredge area is costly and, due to the porous joints of sheet piles, cannot guarantee complete particulate containment. Finally, while the referenced physical barriers provide some containment, once the barriers are removed the resuspended particles that were temporarily contained are free to migrate from the dredge area.
Although some advancement has been made in bucket or vacuum dredging, little progress has been made in developing physical barriers that effectively contain resuspended particulates or soluble contaminants during a dredging operation.
Smith, in his U.S. patent, Dredge Environ Protection Assembly, U.S. Pat. No. 4,386,872, Jun. 7, 1983, does describe an approach for temporarily isolating an area within a waterway during a dredging operation to prevent disturbed sediment from spreading. Smith proposes the use of a barge-mounted assembly where the barge (an open hopper barge) system is floated over the area to be dredged and a series of separate large side panel members (preferably xe2x85x9c inch thick steel, 24 feet long, and 18 feet high) are lowered from the barge to the river channel to isolate the dredge area. Smith""s invention requires that the area within the panels be pumped dry prior to the excavation. Smith further describes the use of panel support columns that would be lowered to a depth below the bottom necessary to resist the water pressure that would be exerted on separate side panel members. Smith""s invention provides for the use of a series of support column caissons (preferably 4 feet in diameter) that would be lowered to the bottom to support the entire vessel when the inner panel area is dewatered. Smith describes these caissons as xe2x80x9cstiltsxe2x80x9d to support the barge above the channel. Smith makes no mention as to how these caissons would support the barge on a soft muddy foundation, but does state that the columns would be lowered to a depth below the bottom surface of the waterway that would be required to support the barge. Smith describes in his patent how the panels would be lowered and set up end to end until they reach the bottom with fluid-tight seals located between the side panels and the panel support columns. Smith describes these fluid tight seals as xe2x80x9crubber sealing stripsxe2x80x9d to prevent infiltration of water through the panels into the dredge area.
Although Smith in his invention proposes equipment to physically separate the dredge area from the ambient water environment, Smith""s invention lacks practical utility. His plan to pump the area dry would exert significant hydrostatic pressure on his proposed sliding panel wall and its support structure. Smith fails to recognize that continuous pumping will almost certainly be required to maintain his dredge zone in the dry condition that his invention requires, and he does not address the disposition of the water that would be pumped from his dredge zone, which can be expected to contain contaminants.
Smith""s invention requires caissons to support the entire weight of the barge. This would necessitate that his support caissons be driven to a firm foundation such as bedrock. Such a foundation could be well below the bottom of the channel, further complicating the practicality of the invention. Smith does not describe any means to drive the panels below the initial placement depth in the event it becomes necessary to excavate deeper to remove additional contaminated sediment during a dredging operation. His proposed use of rubber sealing strips sandwiched between his panel support columns and panels, to prevent water infiltration, is theoretically plausible, but Smith fails to address how these strips would be attached nor how they would be sufficiently secure to withstand the abrasive forces they would encounter during deployment and redeployment of the side panels (if his system is to be mobile as implied). Smith does not address how the joints between multiple panels that lay one upon another, if needed, would be sealed.
Smith implies in his patent that an inclined non-horizontal bottom channel, which would result in a gap between the bottom panel and the inclined bottom surface, could be mitigated by attaching one or more extension panels to the bottom panel to block the flow of water. Smith states that such lower extension panels could be attached to the upper panels with bolts. Smith fails to address how these gaps would be identified, how the extension panels would be sized, and how bolts would be attached. One can only assume that Smith is considering the use of divers to bolt extension panels to all locations where gaps in the bottom seal are identified. Even so, Smith fails to address how these lower extension panels would be made watertight. Smith""s invention fails to account for debris such as logs, rocks, etc. that will invariably be encountered at the bottom of the channel at most dredge locations, preventing a side panel from fully penetrating the subsurface, thereby resulting in additional gaps under the panel. Large gaps would result in an inability to pump the inner dredge area dry, which is a key feature of Smith""s invention. In summary, the authors believe that Smith""s invention lacks the means to make it a practical mobile vessel for removing contaminated sediments in a dredging application.
Other inventors have focused their efforts on the development of physical barriers in the marine environment to contain spills or to provide dry areas for construction.
Santamaria, in his U.S. patent, Barrier for Water Treatment, U.S. Pat. No. 6,089,789, Jul. 18, 2000, describes a curtain barrier system that can be deployed to isolate two bodies of water for the purpose of treating the water contained on one side of the barrier without mixing of the two water bodies. Santamaria""s invention includes two flexible water impermeable curtains, similar to conventional silt curtains, that are deployed vertically, joined to a bottom web that contains ballast material to hold the barrier to the lagoon bottom and end anchors (on shore) to prevent the movement of the barrier.
While Santamaria describes in his invention a system to isolate two water bodies, Santamaria does not relate his system to a contaminated sediment dredging operation. His invention is intended to provide for the treatment of water on one side of an area of a lagoon that is separated from the remaining lagoon locations. Santamaria""s invention is similar to commercially available silt curtains that are held in place by a ballasted bottom and a buoyed float at the top. Santamaria describes features of his impermeable flexible curtain that differentiate it from silt curtains used in commercial applications. While Santamaria""s invention does provide some measure of separation, it cannot be used in the center of a channel where it cannot be anchored, nor would it be effective in sealing in a location where there are currents, and as such its application is primarily intended for a quiescent lagoon.
Strange, in his U.S. patent, Marine Pollution Containment Device, U.S. Pat. No. 4,889,447, Dec. 26, 1989, describes a containment barge consisting of two self-propelled semi-circular hulls that can be oriented in a manner that contains marine spills. The device described by Strange is designed as a rapid deployment device and provides for the mobile containment of oil spills at the surface. It does not extend vertically to the bottom of a channel, which would be necessary to address contaminated sediment dredging containment issues.
McClellan, in his U.S. patent, Mobile Cofferdam, U.S. Pat. No. 5,277,517, Jan. 11, 1994, describes a mobile cofferdam vessel that consists of a watertight rectangular shell that can be filled with water to submerge the cofferdam vessel in place, or have the water evacuated to float the vessel. The submerged vessel is designed to replace conventional stationary cofferdams that establish a zone that could be dewatered for use in constructing on the bottom of a waterway. McClellan describes the use of a pump to dewater the interior of the cofferdam to define a dry working environment.
McClellan""s invention is intended for construction of elongated structures (such as tunnels or piles) below a waterway where a physical barrier is submerged by sinking a physical structure at a desired location and dewatering the area inside the physical structure so that construction can take place. The application is not designed nor intended for contaminated sediment excavation and removal.
In contrast to the prior art, the invention being disclosed herein relates to the development of a system that isolates a dredge area and produces a negatively pressurized control zone into which mechanical or hydraulic dredging apparatus can be placed and used to remove contaminated sediment. The invention does not require the dewatering of the subject dredge area. The invention does not require the use of large columns to support a vessel. The invention is not susceptible to large gaps in the barrier that would result in major water infiltration, and the invention provides a practical means to deploy and redeploy the vessel during continuous dredging operations along a waterway. As will become apparent, the maintenance of this negatively pressurized control zone ensures a secure dredging operation that prevents the release of both particulate and soluble contaminants, and can be readily monitored and evaluated to establish or certify that the dredging operation has met its specified objectives. These features are not included in the prior art. In addition, the inventors have included a new method for managing dredge solids by securing and stabilizing said solids at the dredging site to minimize the opportunity for spills associated with conventional solids management approaches.
It is therefore an object of the invention to remove contaminated sediments from the bottom of lakes, rivers, reservoirs, and other water bodies in a manner that prevents the release and transport of particles or soluble contaminants from the dredge site during the dredging process.
It is also an object of the invention to establish boundaries of a dredge control zone where the excavation of sediments can occur in a contained manner.
It is a further object of this invention to make such a control zone mobile to permit redeploying the invention along a contaminated waterway.
It is a further object to establish a negative differential pressure between said dredge control zone and the ambient water environment.
It is yet another object of the invention to establish such a negatively pressurized control zone by deploying a sealed, vertical barrier wall beneath a marine vessel and by pumping water from the inner volume of the control zone to establish a differential pressure gradient between the control zone and the ambient water environment.
It is also a further object of the invention that such a sealed, vertical barrier will consist of sheets and sheet pile joint seals to respectively define the surface area and volume of the control zone, and to reduce the permeability through the joints of the sheets and hence the rate of flow required to establish said negatively pressurized control zone.
It is still a further object of the invention to treat water collected from the control zone during the pumping process with a polymeric membrane filtration treatment system to enable the discharge of this purified water back into the ambient environment.
It is yet a further object of the invention that such a polymeric membrane filtration treatment system be incorporated into a self-contained mobile water treatment vessel, to avoid the need for land-based facilities.
It is also a further object of this invention to incorporate mechanical and/or hydraulic dredging equipment for use within this control zone to improve the effectiveness of the subsurface cleanup.
It is a further object of this invention to provide the means to contain all resuspended contaminants within said control zone until such time as the dredge area can be certified as complying with the desired specification of the cleanup.
It is a further object of the invention to incorporate a dredge solids stabilization and transportation vessel that can manage the contaminated solids that are collected in the process and transport such solids to an off-loading facility in an environmentally sound manner.
In keeping with these and other objects of the invention, a system is described that provides for the containment of particles and contaminants by the use of a specially designed marine vessel that establishes a negatively pressurized dredge control zone that can be used with either mechanical or hydraulic dredging equipment to remove contaminated sediment. The physical boundary of the control zone is established by deploying a vertical barrier wall in the form of sheet piles and impermeable sheet joint seals located within and below an open barge-like vessel.
While the use of sheet piles and impermeable sheet joint seals can assist in establishing the physical boundary of the dredge control zone in which suspended contaminants are contained, the driving force used to ensure that neither soluble nor particulate contaminants are released from this zone during dredging operations is provided by the introduction of a pumping system located inside the control zone. This pumping system withdraws water from the control zone to establish a negative pressure gradient directed toward the inside of the control zone from the ambient water environment. Development of this negative pressure gradient between the ambient water surface and the control zone water surface elevation ensures that all flow through the vertical barrier is into the control zone, thereby preventing the release of any liquid or particulate matter contained within the confines of the control zone into the ambient water environment.
Liquid pumped from the control zone, where dredging occurs, can be expected to contain contaminated particulate matter and, in some cases, soluble contaminants that must be treated prior to disposal into the ambient water environment. To achieve the appropriate degree of treatment for the collected water, the invention makes use of a polymeric membrane treatment system capable of removing particulate matter in the range of 0.01 to 10 microns, and soluble contaminant removal, if warranted.
Most polymeric membrane filtration systems in commercial operation at the present time utilize positive pressure as a driving force to pass liquids through the membrane. In recent years, immersed vacuum-driven hollow fiber membranes have been introduced commercially. An immersed hollow fiber membrane filter operating under negative pressure is a polymeric filter that achieves filtration by drawing water through a thin fiber (membrane) surface into the hollow annular inner core of the fiber. Either pressurized or immersed hollow fiber membrane filtration systems are suitable for use in the subject application.
Once the subject control zone is established, dredging can be undertaken using either mechanical or hydraulic dredging techniques with assurance that any suspended material or soluble contaminants will be confined to the control zone or pumped out to the membrane treatment system for removal. As will subsequently be described, the preferred operational mode makes use of both mechanical and hydraulic dredging equipment to remove contaminated sediments.
To minimize the pumping rate required to maintain a differential elevation and hence a negative pressure gradient between the control zone and the ambient water environment, the inventors have incorporated a sheet pile joint seal into the system. This sheet pile joint seal is designed to reduce the flow of water through the joint between the individual sheet piles. Said sheet pile joint seal consists of a flexible shroud that is deployed from the vessel and slides down the outer portion of the vertical barrier wall to secure each sheet pile joint. The effectiveness of the flexible shroud is enhanced by the established pressure gradient, which acts to force the flexible shroud into the sheet pile joint, thereby reducing the flow of water through the joint.
To provide the means to effect the removal of contaminated sediments, to treat the collected water, and to manage all sediments without the need for land-based facilities in the vicinity of the cleanup, the inventors have developed a system in which all components of the subject invention are adaptable to mobile marine vessels. This includes the specially designed control zone vessel, referred to as a contaminated sediment removal vessel, a membrane water treatment vessel, and a solids stabilization and transportation vessel.
The system and vessels included in the invention are designed to provide mobile platforms that can readily be deployed and redeployed in order to provide for an ongoing cleanup over an extended length of contaminated waterway.
While other inventors have proposed physical barriers for separating a dredging area from an ambient waterway to prevent the escape of contaminated particulates, no prior inventions consider the use of a low pressure control zone that is not dewatered to achieve said objective. No prior invention considers the possibility of treating water pumped from said control zone. No prior invention provides the means to remove soluble contaminants that may be released during the excavation. No prior invention makes use of sheet piles deployed from a mobile vessel as the vertical barrier wall with the means to penetrate the channel bottom and bottom debris (e.g., logs and rock) if necessary to achieve a bottom seal. No prior invention provides for individual sheet pile joint seals that can readily be deployed to the desired depth, and can isolate and assist in sealing the low pressure control zone to prevent the outflow of contaminants and reduce the inflow of water into said zone. No prior invention provides the means to readily deploy and redeploy a vertical, low permeability barrier in a practical manner. No prior invention provides the means to manage solids without land-based solids handling facilities near the dredge site.